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Determining genetic fatherhood is very important in my worldbuilding experiment for lots of reasons, one being succession, but regardless it's the genetic component that's important But it's possible that people don't know who the father is of their child as well. Note that this is not a medieval European world. I specified tech for a reason as that's the only concept that carries over. The concepts of European royalty or culture really don't apply at all.

Given the idealized restrictions below, is there anything that can be done with medieval level technology that can determine with near 0% false positive who is the father given a set known amount of candidates, or if there is a false positive, you know there's a false positive and the test is still useful?

  • Medieval level tech, is anything that was already available or could reasonably be available given aristocratic resources and modern knowledge quickly as in not 200 years from then (so you couldn't easily create microelectronics, you could however manufacture a spinning device with spinning glass tubes full of liquids with cork caps, even if it had to be manually turned by a person, as an example).
  • A false positive is when a single candidate is tested positive for being the father, but it wasn't them. When more than one candidate tests positive, it's not a false positive, that's inconclusive.
  • This is not a magic world.
  • This genetic fatherhood.
  • Assume neither candidates nor mother lie in this scenario.
  • All candidates and mother are cooperative.
  • The father can be guaranteed to be in the list of candidates.
  • Man-power is not much of an issue, though I don't know if it's relevant.
  • Test has to use medieval level technology, so assume you can make glass, work iron etc...
  • Despite having medieval tech, this does not take place in medieval Europe, there's no need to tie customs there to an answer.
  • A long the same lines, the test can be used along side reasonable original societal customs to aid testing. Maybe there's an invariant in society that means X can't be the father etc... But this can't prohibit non-monogamous relationships in general, otherwise the problem basically solves itself.
  • The test has to be useful. So maybe you could use things like phenotypical traits, but if it mostly always boils down to a few people anyway, and barely ever narrows it down, it's not useful. Useful would be something like 30-50% of the time you can make a pretty strong assertion of the father under some average bounds of number of candidates (like 5 or less).
  • Test has to have a low false positive rate. If it only works 50%-ish the time, then fine, but those times it better be pretty accurate.
  • Alternatively, as long as it's still useful, as long as you know there's a false positive, so you can call the test "failed", that also works.
  • The test can take long periods of time, but ideally (not a hard requirement) no more than 5 years.
  • The test can assume some medical knowledge we know today, such as blood types.
  • If it were possible to do limited genetic testing with said medieval technology, that would be a reasonable solution.
  • Assume everyone are historically from the same place, so there's no "obvious" physical markers with respect to people from two completely different ethnic backgrounds making this work some significant amount of time.
  • near 0% does not mean 0%, so 2% false positive may be acceptable, I didn't want to pin point a specific number.

EDIT:

  • Phenotypical Book-keeping is possible, as in physically recording physical phenotypes that can be observed for each individual, from multiple generations prior (so you know a set number of specific phenotypes from parents of each individual, and parents of parents, and so on and so forth for several generations, and those people don't need to be alive for you to know this information, it's recorded).

EDIT2:

I have a new point that may help that didn't violate the above (which means this isn't really a "new rule", rather it serves to help answerers), but people are getting real stuck on way to euro centric world view of things, to the point where the top voted question earlier quite blatantly violates my entire question and doesn't answer it at all, plus assuming a marriage structure and civilization wide monogamy, or assuming that if there isn't monogamy it must be happening the same day constantly.

  • Both the mother and men know who and when they had relations with and when (assume this knowledge is always accessible, as if they recorded it).

  • When the father is ambiguous, it's not very common to have had relations with multiple different partners within the same day, and also not that common within the same week. The chance of different partners increases with time period when the father is ambiguous. If time can be used, there are accurate records to help.

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    $\begingroup$ "Assume neither candidates nor mother lie in this scenario." Well, if the mother don't lie, just ask the mother $\endgroup$
    – Kepotx
    Sep 8, 2022 at 9:50
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    $\begingroup$ @Kepotx This doesn't work so well if the lady got an affair with two men one day apart; She won't be able to tell which one the kid is from exactly, even if she confessed she cheated on her husband. $\endgroup$ Sep 8, 2022 at 11:40
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    $\begingroup$ @Tortliena Actually, this helps a lot, I feel like an idiot. The mother knowing who she did what with and when, and so do the men, and they all know the exact date, it considerably lowers the number of men who possibly can be the father, especially given how fixed most gestation times are. It's fine If most of the time when the father is ambiguous, it wasn't different partners within the same week. $\endgroup$
    – Krupip
    Sep 8, 2022 at 13:45
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    $\begingroup$ @Tortliena This is literally the plot to Mamma Mia! $\endgroup$
    – Dragongeek
    Sep 8, 2022 at 14:54
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    $\begingroup$ @stackoverblown The entire stack exchange system is intended to be a collection of high quality answers. Some deference is given to the OP by allowing them to accept an answer to indicate that it's particularly useful to them. In our help center we specifically discourage and reserve the right to remove signatures taglines and other chitchat. Adding OP specific commentary unrelated to the question makes it harder to read and less useful as a reference to others. $\endgroup$
    – sphennings
    Sep 8, 2022 at 17:43

16 Answers 16

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In the Middle Ages they had not one but two absolutely certain paternity tests.

  1. If the mother was married, then the husband of the mother was the father of the child, according to the legal maxim that what grows in a man's garden is his. This extended to one year after the death of the husband.

    Only in highly specific (such as the husband being far away for at least a year) or else extremely rare circumstances was it possible to argue against the result of the marriage test.

  2. If the woman was not married then the child had no father at all, unless the man explicitly recognized the child as his. Again, once this was done it was very hard to undo. Note that in some places at some times this was not possible.

That's it. It was a simpler world, and simple facts were simple.


P.S. The question asks how to "determine with near 0% false positive who is the father". The question does not ask how to determine who is the sire. That would have been a different question.

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    – L.Dutch
    Sep 11, 2022 at 20:40
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Given medieval technology there is no reliable paternity test. It wasn't until the 1960s when genetic testing to determine paternity became possible. HLA testing had a 80% success rate and couldn't distinguish between close relatives.

If you allow for an early 20th century knowledge of blood types and their heritability, you can compare the blood type of the child and potential father. This methodology does not prove paternity. At best it allows for the exclusion of ~30% of the population who couldn't possibly be the father. As an example two parents with type O blood can only produce a type O child, and two parents with type B blood can only produce a child with type O or B blood.

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    $\begingroup$ You can conclude that anyone with type AB blood cannot be the father, but a guy with type A or type B could be heterozygous for O and still be the father. $\endgroup$ Sep 8, 2022 at 4:48
  • $\begingroup$ @LoganR.Kearsley Corrected. Thanks. $\endgroup$
    – sphennings
    Sep 8, 2022 at 4:56
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    $\begingroup$ Can a whole list of inheritable traits combine to make something semi-useful if close relatives aren't candidates? You know, eye colour, tongue rolling, etc? In conjunction with blood type? Obviously, definitive proof is a no go. I wonder how well they can do with some umderstanding of genetics and close observation. $\endgroup$
    – user86462
    Sep 8, 2022 at 11:24
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    $\begingroup$ @SeanOConnor As a worldbuilder you can have people determine paternity however you want. In the perpetual letdown of the real world it's rare to have objectively measurable traits with simple, well understood heritability, that have high variance within a population. But we're worldbuilders! We don't need to submit to the boring details of mundanity! We can build a world where the honorable house of imperial midwives has learned the Mendelian ways and through careful examination, and consulting the sacred squared can reveal the secrets of lineage, tracing back through the generations. $\endgroup$
    – sphennings
    Sep 8, 2022 at 14:32
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    $\begingroup$ @vsz As is the case with every subject, genetics is far more complicated than the simplified version we learned in school. The idea that blued eyed parent cannot create a brown eyed child has been debunked. $\endgroup$
    – sphennings
    Sep 8, 2022 at 21:04
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Short answer: nope

Longer answer: Too much science and technology into a reliable test for what you can do in medieval times. And stay assured that, considering how picky were monarchs in keeping their bloodline on the throne, if anything could have be done, it would have been done.

A long, long time ago, before Watson was a crick in anyone’s neck, at the beginning of the 20th Century, the question of paternity had no scientific answer. The “presumption of paternity” rule, accepted in the legal system of England 500 years ago, stated that unless a husband could prove he was “sterile, impotent, or beyond the four seas bordering the kingdom” at the time of conception, then he was legally the father of his wife’s child. Today, DNA analysis allows us to determine paternity with almost 100 percent certainty.

Blood typing was the first scientific technique utilized to help identify biological fathers, despite its limitations. If you were paying attention in junior high you might remember a chart describing the possible combinations between the four blood types: A, B, AB and O. They are classified by the presence or absence of A proteins, B proteins, and Rh factors (which determine positive or negative status, but aren’t as important for deciphering paternity) on the surface of red blood cells. It’s relatively simple. For example, if the presumed father is type A and the mother is type O, then the child couldn’t be type B. (Admittedly, this isn’t the most effective application of blood typing.) Determining ABO blood types based on the A and B proteins on the surface of red blood cells can reliably prove that a man is not the father, but can’t reliably prove that he is the father.

Things changed in the 1980s after disco died and DNA testing hit the scene. DNA can indicate paternity with 99.99 percent accuracy. In the early years of DNA testing, technicians needed a blood sample from all three parties for a simple matching game called Restriction Fragment Length Polymorphism. The technicians would isolate one long string of DNA from the father, one from the mother, and one from the child. To each sample they stirred in some enzymes, which chewed up the DNA into a bunch of uneven fragments. Any fragment from the child’s DNA should be the same length and size of a fragment of either mother or father’s DNA. If roughly half of the fragments match the man in question then he is the father!

In the 1990s, paternity tests got even easier – and accessible to daytime talk show hosts – thanks to Polymerase Chain Reaction (PCR), which requires a quick cheek swab from mother, child and presumed father. PCR makes billions of copies of the DNA, so only a tiny original sample is needed. Lab technicians look at 16 specific fragments. Eight should match the father and the other eight the mother.

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    $\begingroup$ "Considering how picky were monarchs in keeping their bloodline on the throne, if anything could have be done, it would have been done" My scenario isn't theirs. There's the possibility of things being possible that weren't then: Society is always cooperative. You can use societal norms to help. Knowledge from today's time period about medical things like genetic and blood types exist. For example, maybe a certain ear shape is paternal, or that you can do protein testing at that tech, take a variety of very small measurements (don't know if would work) $\endgroup$
    – Krupip
    Sep 8, 2022 at 2:58
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    $\begingroup$ To determine paternity between two identical twin candidate fathers, the full genome must be sequenced so that determination can be made by identifying mutations that occurred in the candidates after those candidates themselves were conceived. An attempt is made to identify the mutations that are shared with the offspring, who also needs their full genome sequenced. This is sketchy and difficult even by today's technology. Machines that can fully sequence DNA to find individual mutations are far beyond medieval technology. $\endgroup$
    – Wyck
    Sep 8, 2022 at 3:35
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    $\begingroup$ @Krupip You asked for definitive, without being able to compare genetic sequences, definitive is impossible. You're welcome to say that in your world a phrenological examination can be used to determine paternity with whatever level of accuracy you want. You could also say that in your world the mother always knows the paternity of her children or certain crystals glow when held by a father and their child. It's your world, you can make up whatever facts about it you want. The only limitations are those you choose to impose on yourself. $\endgroup$
    – sphennings
    Sep 8, 2022 at 3:39
  • $\begingroup$ @sphennings: one existing urban fantasy has werewolves instantly and magically know when they have conceived. $\endgroup$
    – jmoreno
    Sep 8, 2022 at 21:22
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    $\begingroup$ @jmoreno Such a world would have interesting narrative potential to explore. Imagine how less colorful such a story would be if it restricted itself to only what is possible in the real world, no werewolves, no instant knowledge of conception. Fantasy without the fantastic would be dull indeed. $\endgroup$
    – sphennings
    Sep 8, 2022 at 21:27
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it depends on how your genetics work

You said that your world has medieval technology but that it is not necessarily our medieval past. This opens up the possibility that, in your world, genetic traits have a different way of expressing themselves.

If the presence or absence of visible phenotypes is a more reliable indicator of genetic heritage than in our world, then it is possible to use vast databases of written records to keep track of which phenotypes belong which lineage. This information can be used then to asserta parenthood with some degree of success.

You can tailor the mechanism for genetic transmission so that the method could be more or less accurate.

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    $\begingroup$ This feels like the best option to do what OP wants with their world. I could imagine an order of clerics who are trained in the arcane arts of measuring noses and skull bumps and calculating heritage. $\endgroup$
    – Wossname
    Sep 9, 2022 at 2:56
  • $\begingroup$ I also think this is the best-fitting option for the original question. However, as for the concrete phenotypes used, I would suggest OP uses something new such as "skin reaction to focused moonlight" or "relative size of ring finger vs index finger". Rationale: in our world, measuring noses and other stuff related to skull shape was part of the racist "science" called Phrenology. So some real-world readers who heard of the topic might a) accuse OP of adhering to or promoting racism for our world or b) be triggered and feel hurt. Just sayin'. $\endgroup$ Sep 11, 2022 at 7:26
  • $\begingroup$ I agree with @JensBannmann on this one. Also, I think this has the added benefit of drama. The "test results" take some time to "be processed" since the phenotypes will have to be tested with some degree of methodology and standards. So measurements are taken by "authorized personnel" and then taken to a records library to be processed. Plenty of time for the drama to unfold in the meantime. $\endgroup$
    – urquiza
    Sep 11, 2022 at 12:40
  • $\begingroup$ Which is why I also agree with @Wossname. The Church should be the final arbiter. The "authorized" personnel should be clerics and since the Church is in charge of marriages and records that makes them the perfect choice to be the arbiter of these types of disputes. I agree that determing phenotypes should not be anything like phrenology, but I guess we could sprinkle some visible phenotypes like unusual hair color or eye shape just so that untrained characters can start wondering whether or not to request a test for their kid. $\endgroup$
    – urquiza
    Sep 11, 2022 at 12:42
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Is there a paternity test method given medieval level technology with low false positive rate?

No, hence things like chastity belts and harems and, perhaps more usefully for those who were really serious about ensuring inheritance through unbroken parental lines, those cultures where inheritance of property, titles and other rights flowed exclusively through the maternal line rather than the paternal one, men can be as eligible to inherit as women in such a system but children only inherit from their mothers, any presumed father's property is 'inherited' by his mother and through her to his children's cousins who unlike his own children are provably related to him.

Blood type is worthless accept to exclude a portion of the entire population, around a third I think so statistically if your not the father there's something like a one in three chance a blood type test will match you as a potential father which is entirely useless for 'proving paternity'.

Inherited traits like eye colour (that you may be thinking of with phenotype?) are as useless for proving paternity as blood type, you can exclude some people with some of them, you can't prove a father with any of them, and recessive traits that can potentially lie dormant for generations might even give you a 'false negative' on occasion if you don't really understand them.

If you are limiting yourself to a medieval technology base then there is no plausible test that might be used to 'prove' paternity with any degree of reliability.

If you want the non-monogamous society you've implied men are just going to have to accept they can't know they're the father so you may just have to go with maternal inheritance lines (absent a will men's property might go to their mother, siblings (male or female) if she's no longer living, if no siblings follow the line down from the grandmother to the nearest relative, if none there go back to the great grandmother and work down again, etc) .. aside from that 'magic' is likely the only option.

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    – L.Dutch
    Sep 8, 2022 at 19:36
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In 1931, before modern paternity tests, one could use phenylthiocarbamide (PTC) as a substitute: the compound is very bitter for part of the population, while the others find it completely tasteless.

The ability to taste it depends on the genetic makeup of the taster.

The Wikipedia page indicates that "the genetic penetrance was so strong that it was used in paternity tests before the advent of DNA matching".

PTC cannot be synthetized in a medieval setting, but luckily there is an inverse relationship between taster status for PTC and for the juice of Antidesma Bunius, a plant native to southeast Asia and northern Australia (people who find PTC bitter find the juice tasteless, and vice versa).

This means that the juice of that plant can be used as a paternity test too.

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  • $\begingroup$ In combination with timing, this might be enough to eliminate at least some significant percentage of the time when few people are compared, especially if we assume uniform random people in a relationship. $\endgroup$
    – Krupip
    Sep 8, 2022 at 14:32
  • $\begingroup$ This will exclude some candidates in some cases (if mother and candidate father both can taste the juice, but the child can't, the candidate was likely not the father), but not really prove anything. $\endgroup$ Sep 10, 2022 at 0:28
  • $\begingroup$ @Paulo, it was used in real life in the '50s to help determine paternity, so it did prove something. atlasobscura.com/articles/… $\endgroup$
    – Vorbis
    Sep 10, 2022 at 8:55
  • $\begingroup$ As that article says, "It’s entirely possible that many PTC paternity tests from the ‘50s were inaccurate." $\endgroup$ Sep 10, 2022 at 15:16
  • $\begingroup$ @PaŭloEbermann "This will exclude some candidates in some cases" Please read my OP in it's entirety. a 30% success rate of being applicable is fine, and in combination with the other answers, and my idealized societal constraints there's well over that here. $\endgroup$
    – Krupip
    Sep 10, 2022 at 22:21
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One reliable test would be comparing the date of birth to the date on which the mother had intercourse with the potential fathers.

Even though the length of a human pregnancy can vary naturally by as much as five weeks, that's a distribution. Most women give birth pretty close to the expected date, 70% deliver within 10 days of their estimated due date. With enough time separation between encounters (1 month, for example) you could have a pretty reliable idea.

With too little separation (a few hours, a week, a day, simultaneously) this test is inconclusive and you will have to resort to options mentioned in other answers.

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It depends what you mean by "medieval level technology". Some things could be done with the tools available, but they did not have the knowledge to do it. Other things could not be done without the whole "tool chain" of the modern world.

If you mean the former, I think restriction fragment length polymorphism (RFLP) would be doable with the tools available, given enough time (probably some years) to set up the process. I have to admit I am far from certain, so either check or allow a certain amount of hand waving in your story.

Say an experienced laboratory technician from the modern day was sent back to 1400, they would need to:

  • Develop some form of chromatography
    • Agar gel is made from seaweed, and has been made for years as a food product in Asia
    • One could put a charge across it using a Voltaic pile
  • Use chromatography to extract restriction enzymes from the right sort of bacteria. They can recognize Haemophilus influenzae caused pneumonia perhaps.
  • Use a standard technique for isolating DNA from tissue of the child and prospective parents.
  • Apply restriction enzyme to DNA
  • Apply chromatography to the DNA
  • Stain gel with something. Could Ethidium bromide be available? I am not sure.
  • Compare bands of prospective parents to child.
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  • $\begingroup$ It's fair to assume time-traveler bio-medical knowledge. I'm a bit concerned about the Voltaic Pile, though copper should be around, Zinc would be my concern, though it appears as if Zinc was extracted in as early as the 1100s. I'm also confused about the chromotagraphy thing, could you explain that one more? The link seems to show things possible to do with future knowledge, and tech of the time, not super concerned about that. Also concerned about gel staining, but getting access to that could be it's own question if that's the only issue. $\endgroup$
    – Krupip
    Sep 8, 2022 at 14:15
  • $\begingroup$ @Krupip you don't need copper and zinc only. Any two dissimilar metals will do to create a voltage. However, you need to clarify - if "medieval level technology" means "only what was available somewhere in the world at that time" then lots of this won't work. If we posit that it means "it can be built from the technology of the day" then TECHNICALLY anything we can do now is admissible, since everything nowadays was built from the tech of the day, just indirectly... $\endgroup$ Sep 8, 2022 at 16:01
  • $\begingroup$ @MichaelStachowsky added the clarification, let me know if that suffices $\endgroup$
    – Krupip
    Sep 8, 2022 at 19:32
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Speaking as a statistician here: You ask for a low "false positive" rate. I can give you a 0% false positive rate: Always answer "No."

If the answer is incorrect, that is a false negative, not a false positive. In other words, if there are no positive answers, there are no false positives.

And the same thing applies to all methods; lean very heavily on the "This Man is Not The Father" answer, and only answer in the positive if the Man in question admits to being the father, the timing is correct, and the child is a male with identifiable heritable features in common with the man. Don't believe the testimony of the mother, no matter what, or anybody else with a plausible motive to lie.

Without matching on every indicator, it should be rare to hit upon an exact match. After all, the father might have the father, brother or cousin of the accused.

If your answer is 99% "Not the Father", you will achieve the low "false positive" you desire, but likely at the expense of a rather high "false negative".

The way actual medieval society tried to ensure paternal accuracy was by enslaving women, controlling their free movement and dress and forcing them to submit to a culture of religious inculcation making sex for women outside of wedlock a mortal sin, and usually a secular crime punishable by death.

Even that was not fool proof, but brutally subjugating women as the sexual slaves of men and indoctrinating them from birth in a belief that their warden God can see all they do -- that can help reduce paternity errors considerably!

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Depending on when you need the test to be done by, you say "no more than 5 years" and if knowledge is as functionally unrestricted as you're saying, while only technology is restricted...

Using precise timing, the list of candidates can be constrained so that we might be able to use uncommonly known inheritable traits like hair thickness, birth weight, finger length and bitter taste sensitivity, all assessible by the age of 5, to pin down the father?

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  • $\begingroup$ Excellent answer! These sources are great, this is exactly the kind of information I was looking for, in combination with the other answers, there's probably a plausibly a way to get what I want, especially with a bit of handwavy wiggle room with how accurate these phenotypical indicators can be, should give me the kind of tests I want. $\endgroup$
    – Krupip
    Sep 9, 2022 at 3:47
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Not scientifically at least, There simply is no sure way given medieval technology to give A definite answer on the father child. The closest you can get is trying to see what Features like eye color skin color hair color he had in common with Any potential fathers. As you can imagine this was not an exact science. It only worked in Game of Thrones because the Queen had multiple children.

In the absence of Science and Technology Perhaps something more mystical. In the Old Testament A man Who suspected his wife Of adultery could take her to a priest, Have him perform a ritual and then pray for judgment Upon the woman should she Be a secret adulterer in the form of barrenness. Of course whether you think this method was effective or not depends on your own opinions of the Abrahamic God . Nevertheless this is probably the best you can hope for with medieval level of technology.

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How complex you make this depends on how heavily your plot needs to lean on it, or question it.

If the test is so easy & so common-place that anyone can do it for themselves, then it simply becomes an 'accepted fact' in your story.
This is like the difference between telling everybody interstellar travel is common-place & trying to actually explain how it's done. Don't explain, if it's not important to the plot. People have written entire successful collections of novels without ever needing to explain quite significant parts of their 'science'. It 'just works'.
Douglas Adams used a cup of tea as the driving principle behind space travel.

So, potential progeny & father both pee on a yellow crocus. If it turns blue, he's the father.
Done. Simple. It's not 'magic' it's some chemical property of this 'special' crocus that 'just works'. As your population is medieval, they don't know the difference between science and magic anyway, but as this requires no potions, no ritual and can be done by literally anyone, it's science not magic. You don't have to spell out to your audience that the crocus can compare genetic markers… that's overkill.

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Yes, but only indirectly. Consider, e.g., Japan prior to the first arrival of European sailors. Then a ship arrives, and sometime thereafter a (Japanese) woman gives birth to a curly-red-haired child. At the very least it's guaranteed the father was not a native of Japan.

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    $\begingroup$ Only useful in extreme and unusual circumstances, such as that single foreigner from far distant lands with some noticeably different racial phenotype in your example, thus entirely useless for the more general day to day use that the OP has stipulated. $\endgroup$
    – Pelinore
    Sep 8, 2022 at 11:24
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    $\begingroup$ Even in those particular circumstances it would not tell who IS the father, but only who IS NOT. $\endgroup$
    – L.Dutch
    Sep 8, 2022 at 12:13
  • $\begingroup$ Oh I don't know @L.Dutch .. only one European in the whole of Japan in the past twelve months and some girl he's been spending time with pops out a baby with European features? .. that'd be pretty conclusive he was the father for most people 🙂 .. it's an extreme outlier situation though so no practical use for the OP. $\endgroup$
    – Pelinore
    Sep 8, 2022 at 12:39
  • $\begingroup$ But this issue is part of the reason some societies make a big deal about excluding people who do not look like them. When all the males look similar, it is hard to tell that a child was not sired by the person who raises that child. $\endgroup$
    – David R
    Sep 9, 2022 at 14:45
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If the problem is to ensure paternity using modern knowledge but without modern tools, rather than specifically to test it after the fact then I have a few ideas.

One way would be to have a ritual around mating where the couple were isolated for 4 weeks or so around the woman's peak fertility, and only had sex in the middle of it. If a baby was born 9 months later you could be sure of the father, especially if you used the baby size as a proxy for how early/late it was.

I have another, much more grim, method that doesn't work for anyone, but can work for one despotic king who wants to be sure of his paternity and doesn't want to isolate or be isolated with the woman. Select 7 recessive traits controlled by a single gene that the royal lineage exhibits, then kill or exile all male children that exhibit those 7 traits (except for the heir). The king only marries women who exhibit those traits. Any child of a woman the king has slept with that exhibits those traits is >99% certain to be the king's child, even if the woman has slept with other men.

A slightly less grim variation that works for a wider pool of nobility would be for them to insist that all servants, guards etc can only have opposite phenotypes, for any randomly chosen phenotypes, to the male noble(s) in the household, and that the female noble(s) are always accompanied by said guards/servants.

Even if you ignore the infanticide and the guards a noble family could just cultivate a set of unique recessive traits and still have a fairly high degree of certainty that any offspring were genetically related.

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  • $\begingroup$ "insist that all servants, guards etc can only have opposite phenotypes" or just impose an artificial 'phenotype' on them, eunuchs are what you really want here 😁 . cut the balls out after puberty and they might still have some desire to go through the motions but you'll never get any kids out of it. $\endgroup$
    – Pelinore
    Sep 9, 2022 at 12:36
  • $\begingroup$ Yea it's pretty much that, though the noble women get more freedom to socialise with some people, and nobody gets their balls chopped off, so it's an improvement. $\endgroup$
    – patstew
    Sep 9, 2022 at 12:48
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Medieval and older systems what was used:

  1. woman found with a man who was not their husband was banished.
  2. woman found with a man who was not their husband was stoned to death.
  3. man found with a married woman was castrated.
  4. harems; married women were prohibited from meeting men, all work where a man was needed was done by castrated males.
  5. harems; all contacts of a wife with a man was noted by first wife/castrated slave. Pregnancy if there was no record in book was punished by death
  6. harems; any man who enter harem area and was not castrated was punished by castration/death
  7. faith - any contact between unmarried people is a sin punished by death
  8. social - women need care from men and other women after birth. If a woman was found with another man than her husband she would not have this support and was treated as a lowest cast person.

All of above was used, in many mixes, first notes are Babylon times around 3000BC. As you see 7 and 8 are used till today in Christian related countries, 4, 5, 6, 7 and 8 in Islamic related countries. Most was common in use till end of XIX century in almost whole world. To be honest, most of patriarchal system rules are for determining who the father is or to make sure that husband is.

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Well, you did not ask for the test to be science-based and we are here at a worldbuildung SE.

Design the world this way

Just postulate a single real difference of your world with real world: through a simple ritual involving burning the hair of all tested persons and dancing to the tune of Hotel California with a tambourine, the information on the real parents of the child is telepathically communicated to everyone in the radius of 10 feet.

The information is either along the lines of "this man and this woman are biological parents of this child" or "the father has not submitted his material".

The ritual is widely known among different cultures on the globe. It is assumed to be discovered in mid-Paleolithic.

Such an alteration might have some interesting and some strange consequences on the social level, esp. if it is so long-rooted.


As for a scientific method: many talked about phenotype, some talked about taste and distinctive inheritable quirks, but a DNA test in the modern sense is quite impossible in medieval times, as many have mentioned here already.

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